Static control polymers are used extensively throughout the electronics, defense, fiber optics, telecommunications, medical, disk drive, and automotive industries for the protection of sensitive electronic components inside as well as outside of the Electrostatic discharge Protective Area (EPA). ESD engineers or site coordinators, packaging engineers, marketing and procurement groups need to select products based upon a formalized Materials Qualification Process (MQP) to verify a supplier’s technical data sheets or specifications.

Often, ESD polymers are used improperly, and the expectation levels for performance are met with disappointment. Utilization of proper ESD guidelines in packaging selection insures product protection, package integrity and protection from electrostatic discharge. Understanding the available materials and the evaluation process is essential. This two-part article will review the test methods used to evaluate ESD protective thermoformed polymers and describe their relative performance. In Part 1, ESD test methods for material characterization are described; in addition, three polymer classification types for vacuum forming are examined. Part 2 of this article features measurement techniques (combined with results) through a comparative analysis of humidity dependent topical antistats, carbon loaded polymers, and humidity independent Inherently Conductive Polymers (ICPs).

Test Methods And Background

Initially, the U.S. Military was instrumental in requiring adherence to Military Standard 263B-1994, which features test methods for the qualification of materials. The EIA (Electronic Industries Alliance, www.eia.org) and the ESD Association (ESDA, www.esda.org) have developed procedures for material selection. Today, the ESDA leads the effort with ANSI/ESD S20.20-1999, followed by ANSI/ESD S541-2003 (ESD Packaging and Materials Standard), which is an invaluable reference standard for packaging and materials in all industries that incorporate ESD sensitive components.

This article will allow the reader an opportunity to review the various ESD Standard Test Methods and material performance for polymers containing Antistats, Carbon and Inherently Conductive properties:

1. Surface Resistance

2. Non Contact Voltage Measurements for Hot Spots

3. Faraday Cup Measurements for Residual Charge

4. Static Decay

ANSI/ESD S541-2003 assigns specific standard test methods to the product categories being evaluated for ESD compliance. For the purposes of this article, the products were evaluated for their surface resistance properties using ANSI/ESD STM11.11-2001 and ANSI/ESD STM11.13-2004. Please note that both standard test methods employ specialized fixturing to measure the specimens being evaluated.

Terms and Definitions [1]

Resistance of Dissipative Materials: A static dissipative material shall have a surface resistance of greater than or equal to 1.0 x 10⁴ ohms but less than 1.0 x 10¹¹ ohms, or a volume resistance of greater than or equal to 1.0 x 10⁴ ohms but less than 1.0 x 10¹¹ ohms.

Resistance of Conductive Materials: A surface conductive material shall have a surface resistance of less than 1.0 x 10⁴ ohms. Volume conductive materials shall have a volume resistance of less than 1.0 x 10⁴ ohms.

Resistance of Insulative Materials: An insulative material per ANSI/ESD S541-2003 shall have a surface resistance or a volume resistance of greater than or equal to 1.0 x 10¹¹ ohms.

Resistance of Electric Field Shielding Materials: Within the conductive materials classification per ANSI/ESD S541-2003, electric field shielding materials shall have a surface resistance of less than 1.0 x 10³ ohms or a volume resistance of less than 1.0 x 10³ ohms. Other methods may also define the electric field shielding classification.

Surface resistance evaluations in combination with other test methods may be necessary to fully characterize ESD materials. With humidity dependent materials, surface resistance values will “rise and fall” with changes in RH. Furthermore, a static control material can fail if improperly formulated, designed, extruded or thermoformed. Consequently, when ESD materials are found to be insulative, isolated charges called hot spots can occur. The tendency of ESD materials to tribocharge (generate charge by friction) is not a function of resistance alone. Standard test methods described herein represent tools to evaluate materials for their overall performance.

Standard Test Methods and Industry Practices for ESD Polymers

Surface Resistance and Relative Humidity (RH)

Often, humidity dependent ESD materials experience a rise and fall in surface resistance readings when RH fluctuates; 1.0 x 10¹¹ ohms is the standard cutoff for retention of static dissipative properties. In practice, however, a lower cut-off is desirable for packaging materials since dry air may be encountered in the shipping and handling process. In cold and dry climactic conditions, relative humidity can reach 4% or below.

Surface resistance testing may be the most important test in evaluating materials. In 1993, the ESD Association adopted ANSI/EOS/ESD-S11.11-1993 [Rev. ANSI/ESD STM11.11-2001] to replace ASTM D257 (in ohms/square), which measured surface resistivity for D-C resistance or conductance on insulative materials. Surface resistance (in ohms) measurements of static dissipative planar materials are performed using a concentric ring fixture (Figure 1), at 12% +/-3% RH (relative humidity), 230C +/-30C after 48 to 72 hours of preconditioning. Lab-to-lab correlation is facilitated by exposing the specimen to low RH, simulating realistic exposure potential.

Figure 1: Concentric Ring Fixture for surface resistance per
ANSI/ESD STM 11.11-2001

Table 1 [2]: Resistance Classification (in Ohms)

When an ESD material falls outside the measuring range of an ANSI/ESD STM11.11-2001 concentric ring fixture, ANSI/ESD STM11.13-2004 provides a standard test method for the measurement of small profile materials, utilizing a standardized two-point resistance probe. For example, a vacuum formed tray with ridges or valleys not accessible with a concentric ring fixture can be measured using a two-point resistance probe. Figure 2 illustrates the technique employed in measuring a vacuum formed tray. A draw is produced that elongates the “starting thickness” of the substrate to reduce material thicknesses. The draw can be measured using ANSI/ESD STM 11.13-2004.

Figure 2: Detail showing effect of drawing on material structure and properties.

Figure 3: Two point resistance measurement per

ANSI/ESD STM 11.13-2004.

Non-Contact Voltage Measurement

The technique for pin pointing hidden charges (hot spots) uses a non-contact voltage measurement device, as shown in Figure 4. ANSI/ESD S20.20-1999 limits voltage at the workstation to less than +/-200 volts. However, individual industries may need to further limit the allowed voltage, depending on the sensitivity of the components they employ. For example, a disk drive company may limit the voltage to +/-10 volts or less, whereas a microprocessor manufacturing organization may set its threshold at +/-100 volts.

Figure 4: Non-contact voltage measurement of charge

Faraday Cup Measurements per Modification of ESDA Adv. 11.2-1995

A method for measuring residual charge on materials is the use of a Faraday Cup. In this test, the specimen (Figure 5) is dropped into a Faraday Cup to determine the charge (nC) on the object, using Q (Charge) = C (Capacitance) x V (Voltage). Some organizations set less than +/-1.0 nC (nano-Coulomb) as a safe parameter for ESD materials.

Figure 5: The Faraday cup is used for measuring residual charge.

Static Decay

This test measures the rate of decay on a charged isolated object to 10 percent of its original value.[4] Fed Std. 101C, Method 4046 specifies that an object charged to +/- 5000 volts should drain the charge to +/- 500 volts, and from +/-1000 volts to +/- 100 volts in <2.0 seconds.

In addition, this test represents a charged material’s ability to dissipate induced voltage with proper grounding. Military Handbook 263B-1994 states that Federal Std. 101C, Method 4046 has difficulty with materials of complex construction such as laminated foams, multi-layered vacuum formed polymers and items too large for static decay equipment fixturing. To compensate for this problem, the disk drive and semiconductor industries use a modification of Fed. Std. 101C, Method 4046 to evaluate static decay using a charge plate monitor (Figure 6).

Figure 6: Schematic of charge plate monitor measurement.

Polymer Types for a Comparative Study [5]

The development of ESD polymers

One of the earliest problems attributed to static discharge was the explosion of a missile at Cape Canaveral due to removal of a plastic tarpaulin after a rainstorm. To prevent future explosions, Ralph Mondano, a Raytheon chemist, mixed carbon black with polyethylene, and developed “conductive polymer.” At that time, NASA was also working at Cape Canaveral and needed a transparent, “see-through” antistatic curtain.

Richmond Technology found that addition of quaternary ammonium compounds to polymers could produce results in the static dissipative range. A pink dye was added so it could be differentiated from standard commercial polymer films. Dan Anderson developed the first antistatic “pink poly” material that used an antistatic additive to promote the draining of accumulated charge to ground.

Fatty-acid Derived Additives for Static Dissipation [6]

Simple fatty amines used in static dissipative plastics are comprised of a nitrogen atom linked to two hydrogen atoms and a long chain of carbon atoms surrounded by hydrogen atoms. This hydrocarbon chain is derived from a fatty acid and can vary from a few carbon atoms long to twenty or more carbon atoms. Because simple fatty amines need high ambient humidity to perform well as antistatic additives, ethylene oxide can be further reacted with them to produce ethoxylated fatty amines. Ethoxylation gives the molecules two polar alcohol, or OH, end groups. Ethoxylated fatty amines are the additives used in dissipative polymer amine technology.

Amide additives used in dissipative polymers are similarly based on a fatty acid attached to an ethoxylated nitrogen atom. However, in these compounds the carbon atom adjacent to the nitrogen has a double bond attachment to oxygen, or a carbonyl group, instead of two single bonds to two hydrogens. In this case, the molecule is termed an ethoxylated fatty amide. It should be noted that commercial nomenclature “amine free” typically means that an amide additive is used instead of an amine.

Either of the two above types of additives can be used to manufacture dissipative polymers by making a homogeneous blend of additive, antiblock, and polyolefin resin. However, two things must occur for static dissipation to take place. The amine or amide must first diffuse through the plastic volume to reach and wet the surface. This is commonly referred to as blooming. High compatibility between the additive molecule and the resin results in an insufficient amount of amine or amide wetting the surface. Without enough additives on the surface, static dissipation will not occur. On the other hand, insufficient additive and resin compatibility results in too much additive getting to the surface. In this case, the substrate may achieve surface resistance results in the static dissipative range, but the exterior will feel excessively greasy and will attract contaminants when the film makes contact with other objects.

Hydrogen-bonding between the additive and ambient water, i.e. humidity, activates the static dissipative characteristics of plastics made with these additives. Since these antistatic agents rely on the presence of water and the hygroscopic characteristic of the materials, many films lose their electrical properties when subjected to low relative humidities over an extended period of time.

Topical Antistatic Agents [7]

Materials that have a low propensity to tribocharge are said to be antistatic per ESD Adv. 11.2-1995. The antistatic property of a material is not a function of its surface resistance. Large conductive materials may appear to be antistatic since low surface charge concentration characteristics through charge distribution minimizes the generation of electrostatic fields. Thus, antistatic materials charge less because of lower areas of contact, improved charge backflow and reduced friction. An example is insulative polyethylene plastic that utilizes a surface coating of moisture with topical antistatic agents. This mixture transfers to other surfaces so that there is antistat to antistat contact when the two surfaces slide against each other. The moisture layer acts to reduce any charge accumulated by redistributing the charge over the material’s surface.

Metallocenes

Metallocenes are reported to be effective static control additives for polymers [8]. One such metallocene is bis(methyl)cyclopentadianyl cobalt, which is alleged to provide very effective static dissipative characteristics.

A drawback to cobalt metallocenes is their low thermal stability, which restricts their usage to polyolefins or other polymers processed below 200°C. Developmental titanates and zirconates have been shown to be stable to higher temperatures, in excess of 260°C. It is reported that specific combinations of neoalkoxy titanates and zirconates form an internal circuitry through their orientation into alternating bipolar charge layers resulting in a very narrow band gap. Therefore, low resistance to electron transfer is insured [9]. Once formed these oriented layers are said to be stable, non-migratory and strongly colored. Unfortunately, metallocene static control additives are also said to be a very costly method for achieving static dissipative results.

Conductive Filler Materials

Carbon black powder and fibers are possibly the most widely used additive to produce readings that are conductive or static dissipative. Conductive results are achieved by loading resin with conductive metal particles, carbon fibers, or carbon black (carbon powder) to form polymer composites. ESD properties are achieved via the application of a surface conductive coating onto an insulative polymer substrate or through a batch-blending process so that the fillers, loading levels and polymer composites are governed by the conductivity required. Consequently, a static control polymer will either exhibit surface or volume resistance properties.

Surface resistance readings per ANSI/ESD STM11.11-2001 from 1.0 x 10² ohms to less than 1.0 x 10¹¹ ohms can be targeted by selecting the proper metallic fillers, coatings, carbon fibers or carbon powders. The use of longer fibers with lower volume resistance readings (conductivity) will improve the electrostatic shielding capability of the end use product. When the proper amount and distribution of the conductive composite is achieved, a process known as percolation takes place when the electrical resistance drops in a dramatic fashion [10]. Thus, a greater distribution of neighboring conductive fillers will assist in achieving overall electrical conductivity in combination with insulative polymer resins. Carbon black powder is the most efficient filler since the geometry of the particles tentacle outward, achieving better conductivity with less loading.

A drawback to carbon black is that it will slough off, transferring conductive particles that could bridge the gaps of circuit lines and cause shorts. This shedding is unacceptable for cleanroom use if the carbon based technology is not engineered to generate a low particle count. Carbon fibers are less likely to slough; these resins are often used for electronics processing, but not in lower cleanroom classifications. Another drawback of carbon-loaded materials is the lack of light transmission to facilitate visual identification and bar coding. One common method in the evaluation of rub resistance for carbon-loaded materials is the Taber Abrasion Test (ASTM D4060).

Inherently Conductive Polymers [11]

Generally speaking, polymers are insulators. However there is a special class of polymers – the inherently conductive polymers – that have conductivity levels between those of semiconductors and metals. ICPs can be used to achieve very conductive surface resistances or static dissipative readings ranging from 1.0 x 10³ ohms to <1.0 x 10⁸ ohms. A common commercially available ICP is poly (3,4-ethylenedioxythiophene) or briefly named PEDT or PEDOT.

ICPs have the following properties:

1. Wider Range of conductivity

2. Transparency

3. High stability at low RH

PEDOT is supplied as a waterborne dispersion of the polymer complex poly (3,4-ethylenedioxythiophene)/polystyrene sulfonate (PEDOT/PSS). The dispersion consists of submicrometer sized gel particles which upon drying form a continuous, conductive, and transparent film.

PEDOT/PSS is a raw material formulated by end users into a coating for plastics, glass and other substrates. These coating formulations, or recipes, have been optimized for individual substrates, such as PETG (glycol-modified polyethylene terephthalate), APET (amorphous polyethylene terephthalate), PVC, polycarbonate, glass, etc. at different wet film thickness and surface resistance readings (Table 2). The PEDOT/PSS-based coatings may be applied by conventional coating methods such as brushing, spin-coating, printing processes, spraying, dipping, and roller-coating techniques. Other binding agents may be added. Unlike traditional antistatic coatings, the PEDOT/PSS coatings are physically dried in an oven at temperatures between 800C to 200°C. The drying time depends on coating thickness, temperature and air humidity and should be determined by appropriate application tests. Typical applications on plastic surfaces for PEDOT/PSS or for other ICP materials are:

1. Electronic packaging (carrier tapes, trays)

2. Plastic electronic component housings and windows

3. Cleanroom packaging

Table 2

Part 2 of this article will highlight several ESD test methods that measure and compare materials in the antistatic, carbon-loaded, and Inherently Conductive Polymer (ICP) types. g

Notes

1. ANSI/ESD S541-2003 [Packaging and Materials Document]

2. Modified Version of Table 1, Page 5, ANSI/ESD S541-2003

3. ANSI/ESD STM11.11-2001

4. From Fed Std. 101C, Method 4046

5. Editing by Dr. Jill Simpson, H.C. Starck

6. Source: Fowler, S., “Packaging in the 21st Century”, ESD Journal March 1998

7. Section from Wayne Tan, MSEE, AMD, at ESDiscovery 1998

8. U.S. Patent 4,715,968, G. Sugarman and S.J. Monte; Kenrich Petrochemicals, Inc.

9. “Unfilled Plastics - A New Application for Coupling Agents”, S .J. Monte and G. Sugarman, SPE RETEC, PMAD Division, November 1986, Akron, Ohio

10. Robert B. Rosner, 3M Electronic Handling & Protection

  Division, Austin TX

11. Jill Simpson, Ph. D, H.C. Starck

12. Trade name BAYTRON® -   H.C. Starck

References

1. Jill Simpson. Ph. D, and H.C. Starck Bayer Material Science (Courtesy of special run vacuum formed ICP sheets and Illustrations)

2. “ESD from A to Z,” John Kolyer, Ph. D, and Watson, 2nd Edition

3. Wayne Tan, MSEE ESDiscovery 1998

4. “ESD Considerations for Cleanrooms,” A2C2 Cleanroom Magazine, June 2005, R. Vermillion and C. Resurreccion

5. Fowler, S., “Packaging in the 21st Century,” ESD Journal, March 1998

6. “Conductive Materials for ESD Application: An Overview,”  ESDA Proceedings 2000, Robert B. Rosner, 3M Electronic Handling & Protection Division, Austin, TX

7. Mil Handbook 1686C-1995

8. Mil Handbook 263B-1994

9. EIA 541-1988 Appendix F, “Triboelectric Charge Testing of Intimate Packaging Materials”

10. ANSI/ESD S20.20-1999

11. ANSI/ESD S541-2003

12. ANSI/ESD STM2.1-1997

13. ANSI/ESD STM4.1-1997

14. ANSI/ESD STM11.11-2001

15. ANSI/ESD STM11.12-2000

16. ANSI/ESD STM11.13-2004

17. ESDA Adv. 11.2-1995

About the Author

Bob Vermillion, CPP, is a Certified ESD & Product Safety Engineer-NARTE and holds a US Patent with several patents pending. One of his recent developments has been approved for a NASA Mars Mission. Bob is a member of the ESD Association Standards Committee and ESDA Packaging Working Group 11 coauthoring ANSI/ESD S541-2003. Bob conducts ESD Seminars in the USA and abroad as well as California State Polytechnic University, San Jose State and Clemson Universities. RMV Technology Group, LLC, a member of the American Council of Independent Laboratories, provides ESD materials testing, training, cleanroom and facility troubleshooting/auditing. Bob can be reached at 925-673-0225 or bob@esdrmv.com.